Six variants of the ROP protein, designed with the aim to analyze by X-ray crystallography loop formation and core packing interactions in 4-alpha-helical bundles, have been purified and a search of their crystallization conditions has been carried out. Five mutants yield crystals that are suitable for medium to high resolution X-ray diffraction studies. For all mutants crystal size, sensitivity to X-irradiation and diffraction limit are correlated to their stability as determined by differential scanning calorimetry, in a manner which is not yet understood in detail.